Considerable effort has been devoted over the past 15 years to the development of effective methods for manufacturing ceramic matrix composites (CMC's). Several approaches with potential for industrial use have been identified. The development of CMC's with high temperature stability theoretically is possible; however, CMC's have not yet been developed for use in extremely high temperature applications, such as multistage nozzles for rocket motors. Such nozzles must be capable of exhibiting high strength even after repeatedly withstanding temperatures of 1600.degree. C. and even higher.
Currently, multistage nozzles are made from tungsten and graphite, which have relatively high melting/sublimation points--a 3410.degree. C. melting point for tungsten, and a 3650.degree. C. sublimation point for graphite. The high temperature strength of a material is proportional to the melting point of that material. If CMC's could be made using materials with higher melting/sublimation points than tungsten and graphite, then the resulting CMC's should be effective alternative materials for making high temperature components, such as multistage nozzles.
Certain metal carbides and metal borides have melting temperatures even higher than the melting/sublimation points of tungsten and graphite. For example, hafnium carbide has a melting temperature of 3890.degree. C. and tantalum carbide has a melting temperature of 3880.degree. C. Metal carbides also exhibit desirable brittle to ductile transition temperatures in the range of 1725-1980.degree. C.
A CMC having a matrix of a refractory metal carbide and/or metal boride and comprising between about 20-30% particulate silicon carbide theoretically would be an ideal alternative for tungsten and graphite in multistage nozzles. Such metal carbides and/or metal borides also might be useful as high temperature coatings for other surfaces which are exposed to high temperatures during operation. In fact, the United States Air Force has recently initiated a new program--Integrated High Pay-Off Rocket Propulsion Technology (IHPRPT)--to incorporate such advanced materials into rocket and space propulsion systems.
Unfortunately, the most widely used method for making CMC's--chemical vapor infiltration (CVI)--is slow, complex, and has many inherent difficulties. One major difficulty for high temperature applications is that CVI produces a CMC with substantial residual porosity (15-25%). The greater the porosity, the lower the strength of the CMC.
Polymer infiltration/pyrolysis (PIP) can produce a less porous CMC. However, PIP can only be used to make metal carbide/metal boride CMC's if precursor polymers are developed which will decompose upon pyrolysis or other energy treatment to yield substantially pure metal carbides and metal borides.